Download Photosynthesis

Photosynthesis
6CO2 + 12H20
C6H12O6 + 6O2 + 6 H2O
Overview
• Light-dependent
reactions of
photosynthesis
• Light-independent
reactions of
photosynthesis
• C3 versus C4
metabolism
• Write an essay on the
light-dependent (light)
reactions of
photosynthesis
• Write an essay on the
light-independent
(Dark) reactions of
photosynthesis
• Compare and contrast
C3 and C4 metabolism
Photosynthesis
CO2
ADP
Light reactions
ATP
H2O
O2
NADP+
Calvin/Benson
Cycle
(“Dark
Reactions”)
NADPH
Sucrose
Photosynthesis occurs only in the
chloroplasts
Stroma
Vacuole
Grana
Chloroplast
Palisade cell
Thylakoids
Thylakoid
space
PSII
PSI
The light reactions of
photosynthesis
Plants absorb Light
Chlorophyll a
Chlorophyll b
Carotenoids
Abs
400 Blue
500
Green
600 Red
Wavelength of light (nm)
700
Absorption of light leads to
excited electrons
• When light energy is absorbed by
chlorophyll, an electron is promoted to a
higher (more energetic) orbital. This energy
can be lost by:
– from one orbital to the next with loss of heat
– back to the ground state with loss of light
(fluorescence) and heat
– transfer to another chlorophyll.
Chlorophylls are organised into
photosystems to trap light energy
Light
Special chlorophyll molecule
- primary electron acceptor
Transfer of light
energy from one
chlorophyll to
another
Chlorophylls are organised into photosystems
Cyclic photosphorylation
generates ATP
Stromal side
Fd
PQ
PQ
PSII
Thylakoid space side
P700
PSI
Non-Cyclic photosphorylation
generates ATP and NADPH
Stromal side
NADP+ NADPH
Q
Fd
PQ
PQ
P680
PSII
H20
2H+ & 2e- & 1/2 O2
P700
PSI
Thylakoid space side
H+
Generation of ATP
Stroma
• H+ gradient generated
by:
ATP
– PQ shuttle (H+)
– Splitting of water (2H+)
– NADPH production
(2H+ removed from
stroma)
• ATP generated by an
ATP synthase as H+
ions flow through it
H+
ADP
+Pi
H+
H+
H+
H+
H+
Light-independent reactions
The Calvin/Benson cycle
Carbon
Phosphorus
Ribulose biphosphate
CO2
3-phosphoglycerate
ATP
ADP
ADP
1,3 diphosphoglycerate
ATP
NADPH
Sugars
G-3-P
NADP+
Glyeraldehyde-3-phosphate
C4 metabolism
C3 and C4 metabolism
• Inefficient as RUBISCO can act as both a
carboxylase and as an oxygenase.
• Oxygenase activity leads to loss of carbon
that has already been fixed.
• Some plants have an alternative pathway
where CO2 is first fixed into C4 organic
acids (C4 pathway) and then liberated later
to undergo the Calvin/Benson cycle (C3
metabolism).
C4 metabolism
• 2 types
• C3 and C4 metabolism are separated in
space (different cells) e.g. sugar cane
Characteristic aggregation of cells around
the vascular bundles - Krantz morphology
• C3 and C4 metabolism are separated in time
(same cell) e.g. pineapple.
Comparison of C3 & C4 leaves
• C3 - note the lack of
chloroplasts in the
bundle sheath
• C4 - note the extensive
chloroplasts in the
bundle sheath
CO2
OAA (4C)
PEP (3C)
ADP
ATP
Separation
in space
Mesophyll cell
surrounding
bundle sheath
Pyruvate (C3)
CO2
C3
sugar
Vascular tissue
Bundle sheath cell